[Deadbug]

I had a Max that did this. He's territorial. We'd call mine "Max-hole" because of this.

You're going to have to teach him to "go to his house" when the doorbell rings. It's the only way to protect him and the person on the other side at the same time. Unfortunately, I don't think you'll ever get him reliable not to attack a stranger.

Have someone ring the doorbell and just after tell him to go to his house. Make sure it's someone he knows and loves. As soon as you put him in his house, give him a treat. Keep doing this until he gets it.